The principle governing the operation of most magnetic read heads is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance or MR). Magneto-resistance can be significantly increased by means of a structure known as a spin valve where the resistance increase (known as Giant Magneto-Resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of their environment.
The key elements of a spin valve are illustrated in FIG. 1. They are seed layer 11 (lying on lower conductive lead 10) on which is antiferromagnetic layer 12 whose purpose is to act as a pinning agent for a magnetically pinned layer. The latter is a synthetic antiferromagnet formed by sandwiching antiferromagnetic coupling layer 14 between two antiparallel ferromagnetic layers 13 (AP2) and 15 (AP1).
Next is a non-magnetic spacer layer 16 on which is low coercivity (free) ferromagnetic layer 17. A capping layer such as lead 18 lies atop free layer 17. When free layer 17 is exposed to an external magnetic field, the direction of its magnetization is free to rotate according to the direction of the external field. After the external field is removed, the magnetization of the free layer will stay at a direction, which is dictated by the minimum energy state, determined by the crystalline and shape anisotropy, current field, coupling field and demagnetization field.
If the direction of the pinned field is parallel to the free layer, electrons passing between the free and pinned layers suffer less scattering. Thus, the resistance in this state is lower. If, however, the magnetization of the pinned layer is anti-parallel to that of the free layer, electrons moving from one layer into the other will suffer more scattering so the resistance of the structure will increase. The change in resistance of a spin valve is typically 8-20%.
GMR devices may be designed so as to measure the resistance of the free layer for current flowing parallel or perpendicular to its two surfaces. The former is referred to as a CIP (current in plane) device while the latter is called a CPP (current perpendicular to plane) device.
For read head application, a CPP spin valve structure grows on a NiFe bottom shield directly, which is different from a CIP GMR reader sensor. In the latter case, the spin valve structure was grown on an insulation gap layer, typically alumina. Since NiFe is an ordered structure, it reduces the effectiveness of the seed layer/AFM combination that exists in the CIP GMR case. The present invention discloses a seed layer that is more suitable for CPP GMR spin valve structures.
A routine search of the prior art found the following references to be of interest:
In U.S. Pat. No. 6,624,985, Freitag et al. disclose a Ru/Si and NiFeCr seed layer for PtMn. In US 2004/0179311, Lin et al. (Headway) teach that a typical seed layer is Ta and NiCr. US 2003/0030945 (Heinonen et al.) shows a NiFeCr or Ta seed layer while US 2002/0051330 (Heijden et al.) shows a seed layer that may comprise Cu.